The polymers of interest to this invention are substantially fluorinated and have pendant chains containing sulfonic acid groups or derivatives of sulfonic acid groups. The sulfonic acid groups exhibit extraordinarily high acid strength compared to sulfonic acids that are not fluorinated. Therefore, these materials are very useful as strong acid catalysts and have been shown to be effective in catalyzing many different reactions, such as: hydration of olefins and epoxides, dehydration of alcohols, alkylation and acylation of aromatic hydrocarbons, isomerization and alkylation of paraffins, nitration of organics, pinacolone rearrangements, esterification of carboxylic acids, hydrolysis of carboxylic acid derivatives, and Diels-Alder condensations.
Fluorocarbon polymers with sulfonic acid pendant groups have advantages over other types of acid catalysts in that the fluorocarbon portion gives extraordinary chemical and thermal stability, as well as almost complete insolubility in most systems. Therefore, the polymer can be used as a heterogeneous catalyst and can be recovered very easily and reused. Since the reaction occurs on or near the catalyst surface, the amount of surface area must be maximized to obtain optimum efficiency, defined as activity per unit weight of catalyst. This is especially important when considering fluorocarbon polymers because of their relatively high cost. One factor that has prevented wide-spread use of these materials as catalysts is their high cost. Their manufacture requires more specialized technology and a larger capital investment than for conventional ion exchange catalysts. It is for this reason that the catalytic efficiency of such a product must be maximized. As noted above, the catalytic efficiency is defined as the amount of product produced divided by the amount of catalyst used. One technique of increasing the efficiency of a heterogeneous catalyst is to increase its surface area, thereby exposing more reactive surface while involving less unused catalyst below the surface. The process of this invention of applying a thin coat of polymer catalyst to a carrier increases the ratio of active surface area to weight of polymer, compared to that for an unsupported polymer catalyst.
In the prior art, increasing the surface area of a fluorocarbon polymer has been accomplished by several methods, all of which have inherent disadvantages. By decreasing the particle size of a solid, the surface area is increased. However, the disadvantages of using a fine particulate catalyst include poor flow dynamics, plugging problems, loss of catalyst include poor flow dynamics, plugging problems, loss of catalyst by entrainment, and more difficult catalyst recovery .
As an example of one prior art enhancement, the fluorocarbon can be extruded into tubing while it is in the thermoplastic sulfonyl fluoride (SO.sub.2 F) form, then converted to the sulfonic acid. Extrusion into tubing requires expensive, specialized equipment and careful handling of the fragile material during processing and reactor assembly. Furthermore, the mechanical strength of the polymer is such that tubing with a wall thickness less than about 0.005 inches (0.125 mm) becomes impractical. This results in only a modest surface area to weight ratio.
The polymer is the thermoplastic sulfonyl fluoride form can be melt deposited onto a solid substrate, and then the surface layer can be converted to the sulfonic acid. This process also requires specialized equipment to form the catalyst to the desired shape of the substrate. Only the portion of the polymer on the surface is used in the catalytic process since the subsurface portion must remain in the SO.sub.2 F form to remain bonded to the substrate. This is an inefficient use of the expensive polymer because the surface area is small compared to the amount of polymer below the surface.
Although it is considered substantially insoluble, dilute solutions of the fluorocarbon polymer in the sulfonic acid form can be prepared. These solutions can then be used to coat supports to make catalyst pellets, for instance. But the process of dissolving the polymer converts it from a substantially insoluble species to a species that is very soluble in many polar solvents. Thus, supported catalysts prepared by this method in the prior art have only limited utility because the polymer redissolves very easily in many solvents.
One aspect of the present invention is the surprising discovery that a polymer having sulfonic acid groups can be deposited onto a support from a solution, as described above, and then annealed at an elevated temperature, thereby rendering the polymer insoluble. This annealing step unexpectedly reduces polymer leach during a reaction, thereby resulting in a more durable and long lasting catalyst. By supporting the polymer on a carrier, the surface area of the catalyst is increased, and this in turn improves the catalytic efficiency and lowers the cost of the catalyst.